Yaris 2005 sudden electrical failure

[sceh]

I had a very close look at the battery terminals and one of them was not ultra-tight. I tightened it.
My conclusion is that the main ECU, like all computers, expects a certain voltage and if there is any dip in the voltage it reboots itself as soon as the voltage is inside the acceptable bounds. A bad connection could reduce the voltage when, for example, the car goes over a bump. It is certain that the sudden cutout is not high-current related but logic related and low voltage. It is unlikely to be any one peripheral ECU having a problem since this is not how the CAN system works and sub-ECU tests have been run to ensure that no single one kills the main ECU. Ergo, the main ECU is the culprit and ergo, the only thing that can kill it is a bad ECU connection or a voltage glitch.
I have seen comments saying it could be the alternator. No chance since all it does is charge the battery and if the battery voltage is low you would notice it at start-up.